OF RESPIRATION". 



217 



and thus the stratum in contact with the vascular membrane 

 is incessantly renewed, and respiration thereby maintained. 



[In the Echinidce (fig. 174), the space comprised between 

 the viscera and the test is filled with water, which is drawn 

 into and rejected from the body by five pairs of mem- 

 branous respiratory tubes, collected into ten tuft-like organs, 

 situated around the circumference of the oral aperture, and 

 opening internally by two perforated pits, as in Asterias. 

 The water thus introduced into the interior of the test 

 Hows along the membrane, covering its surface, and over 

 the peritoneal layer, investing the digestive organs and 

 tubular feet and ovaria by the action of cilia, so that the in- 

 terior of the 

 test of the 

 Echinus is in- 

 cessantly tra- 

 versed by re- 

 spiratory cur- 

 rents, whilst 

 the blood, 

 circulating 

 through 



the 



coriaceous in- 

 tegument, is 

 in like man- 

 ner aerated by 

 currents flow- 

 ing over its 

 surface by the 

 vibrations of 

 cilia. 



In the Ho- 

 lothuria (fig. 

 232), the re- 

 spiratory 

 function is 

 limited to a 

 pair of or- 

 gans formed 

 after a type 

 which attains 





Fig. 232. The anatomy of the Holothuria tnbuio a. 



